This disclosure relates to methods, algorithms and apparatus for enabling unambiguous, high-resolution measurement/computation of the direction of propagation of a traveling wavefront.
It is well-known in the art of measuring direction of arrival (henceforth, DOA) of a traveling wave that the resolution, accuracy and immunity to multipath and multi-signal interference of the measurement are increased by increasing the physical aperture of the receiving sensor in emitter location relative to the receiver, and of transmitting illuminator in receiver location relative to the transmitter (as in radio navigation aids). Said physical aperture may be realized either by a "continuous" structure (a reflector or a lens) that collects incident energy over the extent of the aperture and focuses it onto a receiving "point" sensor, or beams it out of a "point" radiator; or by a spatially spread set of discrete (receiving or transmitting) units which in effect "sample" a physical area or volume. This invention relates to the latter method of realizing a physical aperture, and "aperture" is defined here as the maximum physical lineal separation between members of said discrete set.
This invention relates to discrete aperture-sampling sets of antennas with which the direction of propagation of a traveling wavefront is ultimately derived either from the phase-shift differences between the outputs of the most widely separated pairs of antennas, or by employing the outputs of the various discrete antennas to set up an algebraic system which is then solved for characteristic indicators of said direction of propagation. In this disclosure we shall refer to the phase-shift difference method as paired-antenna interferometry (or PAI, for short), and to the algebraic system approach as the DOA-computing array processing algorithm (or APA, for short) approach.
The baseline length of a paired-antenna, phase-difference measuring interferometer (PAI) is the key parameter for high-resolution and accuracy of direction-of-arrival (DOA) measurement by this means. The baseline length is in essence the aperture of a DOA sensor in which this sensing is based on the phase difference accumulated by the incident wavefront in transit from one end of the baseline to the other. If the path length traversed between these ends is equivalent to more than one wavelength of the incident wave, then the corresponding phase shift will include an integer multiple of 2.pi. rad that will not be revealed by a phase-difference detector. The phase-difference measurement is then said to be cyclically ambiguous. Cyclic ambiguity is resolved in prior art by means of other phase-difference measurements that are also subject to cyclic ambiguity that are performed in parallel between the outputs of additional pairs of antennas separated by judiciously chosen, progressively shorter baseline lengths; or, in cooperative situations that so avail, between the outputs of the same pair of antennas, on components of the same signal that differ appropriately in frequency. As a result, complexity and cost of an interferometer go up with baseline length, largely because of escalating costly provisions for resolving the cyclic ambiguities. The special design requirements and the added initial nonrecurring acquisition cost and later recurring calibration, operation and maintenance costs, of the prior art methods of cyclic ambiguity resolution, set severe limits on affordable or permissible aperture extent (interferometer baseline length).
The technique disclosed herein provides a means for resolving the cyclic ambiguities of the phase difference between wavefronts of the same signal wave at the positions of a pair of antennas that are separated by a baseline length equal to an an arbitrary number of wavelengths of said signal wave by employing a method of directly obtaining a first estimate of the cosine of the direction of arrival (DOA) through a cyclically unambiguous, or non-PAI, measurement. In this disclosure, we introduce the concept of "hybrid interferometry", wherein one opts to employ longbaseline phase-difference-measurement interferometry only for the "fine" measurement of the DOA, and other means for the "first estimate" or "coarse" measurement, that resolves the cyclic ambiguity in the fine measurement.
It is therefore an object of this invention to provide an alternative method and means for resolving long interferometer baseline phase-difference cyclic ambiguities at significant reduction in costs and complexity in comparison with said prior art methods, and thus eliminate the ambiguity resolution requirement of a long-baseline PAI as a primary factor limiting affordable baseline length.
The invention also applies to DOA-computing array processimg algorithms (or APA's) such as those known in the art by the descriptive labels of beamforming, maximum-likelihood, MUSIC (for multiple signal classification) and ESPRIT (for estimation of signal parameters via rotational invariance techniques). Characteristically, all said algorithms involve complex and lengthy computations that inherently start with a search-and-plot procedure to reveal the peaks of a measure or an indicator of signal presence versus DOA. In all of said APA's, the computation load and time would be significantly reduced if additional information is provided to point the way to solutions in the form of at least coarse first estimates of DOA's of some or all incident signals picked up by the antennas in the array.
It is therefore another object of this invention to provide a method for significantly reducing the computation load and time of DOA-computing APA's by employing a method of directly obtaining first estimates of the cosines of the DOA's of some or all of a number of incident signals, and hence restricting the required computations only to the refinement of those estimates.
It is yet a further object of this invention to provide methods and algorithms for estimatimg the cosine of the direction of arrival of a wavefront that are not subject to, or require/involve resolution of, cyclic ambiguity.
These and other objects and features of this invention will become apparent from the claims, and from the following description when read in conjunction with the accompanying drawings.